Very low blood flow, or low "systemic perfusion," is typically due to low aortic pressure and can be caused by a number of factors, including hemorrhage, sepsis and cardiac arrest. When there is a reduced flow of blood from the heart, the body directs a higher portion of blood to critical organs, such as the brain, which will not survive long without a continuous supply of blood, while restricting the flow to less critical organs, such as the stomach and intestines, whose survival is not as threatened by a temporary large reduction in blood flow. Physicians commonly take advantage of this phenomenon by taking measurements in the stomach and intestine to assess perfusion failure.
Assessment of CO.sub.2 concentration in the less critical organs, i.e., those organs to which blood flow is reduced during perfusion failure, is useful in perfusion assessment. Carbon dioxide production, which is associated with metabolism, continues even during low blood flow. Because CO.sub.2 is not rapidly carried away during low blood flow, the concentration of CO.sub.2 increases, which in turn results in a decrease in pH and an increase in partial pressure of CO.sub.2 (PCO.sub.2) in the less critical organs. Therefore, perfusion failure is commonly assessed by measuring pH or PCO.sub.2 at these sites, especially in the stomach and intestines. For examples of catheters used to assess pH or PCO.sub.2 in the stomach or intestines, see, e.g., U.S. Pat. No. 3,905,889 to Macur, U.S. Pat. No. 4,016,863 Brantigan, U.S. Pat. No. 4,632,119 to Reichstein, U.S. Pat. No. 4,643,192 to Fiddian-Green, U.S. Pat. No. 4,981,470 to Bombeck, IV, U.S. Pat. No. 5,105,812 to Corman, U.S. Pat. No. 5,117,827 to Stuebe et al. U.S. Pat. No. 5,174,290 to Fiddian-Green, U.S. Pat. No. 5,341,803 to Goldberg, U.S. Pat. No. 5,411,022 to McCue, U.S. Pat. No. 5,423,320 to Salzman et al., U.S. Pat. No. 5,456,251 to Fiddian-Green, and U.S. Pat. No. 5,788,631 to Fiddian-Green et al.
The measurement of PCO.sub.2 to determine the extent of perfusion failure has commonly been done by threading a catheter through the nasal passage, past the epiglottis, through the esophagus, past the esophageal sphincter, and into the stomach, and sometimes through the stomach and into the intestines. Alternatively, measurement has been conducted in the colon, with a catheter being threaded through the anus. These procedures are obviously quite invasive and can cause harm and discomfort to a patient. Moreover, insertion of the catheter in this manner is also complex and time-consuming.
In U.S. Pat. No. 5,579,763 to Weil et al., applicants described the introduction of a catheter with a carbon dioxide sensor through the nasal or oral passage, past the epiglottis, and into the esophagus so that the catheter and sensor lay within the esophagus. This method can be used to accurately assess perfusion failure by measuring PCO.sub.2 in the patient's esophagus of a patient, rather than in the stomach and/or intestine. Tests showed that measurements of PCO.sub.2 in the esophagus are closely correlated with aortic pressure, and, furthermore, that measurements made in the esophagus are even more closely correlated to aortic pressure than measurements of CO.sub.2 in the stomach. This procedure was advantageous in that the procedure's invasiveness was reduced and CO.sub.2 generated by digestive fluids in the stomach did not affect measurements since the esophageal sphincter blocks such gas. However, the insertion of the catheter still constituted considerable invasion and thus risk of harm to the patient. Furthermore, extension of the catheter extended past the epiglottis exposed the patient to the risk of regurgitation of stomach contents including stomach acids.
In co-pending U.S. patent application Ser. No. 09/160,224, filed Sep. 24, 1998, and in the corresponding PCT Application No. PCT/US98/20118, filed Sep. 25, 1998, the present inventors described a less invasive method for assessing impairment of blood circulation in a patient, such as that in perfusion failure, by measurement of PCO.sub.2 in the upper digestive and/or respiratory tract of a patient. As explained in the aforementioned U.S. and PCT applications, that method involves introduction of a carbon dioxide sensor into the upper digestive and/or upper respiratory tract of a patient, without passing the sensor down through or beyond the patient's epiglottis. The carbon dioxide sensor is placed adjacent a mucosal surface within the upper digestive and/or respiratory tract, e.g., within a patient's mouth, inside a patient's nose, or at a site along the wall of the esophagus.
Patients who require mechanical ventilatory support pose special problems in measurement of PCO.sub.2 using the aforementioned methods. By "mechanical ventilatory support" is meant an apparatus that supports the ventilatory function of the respiratory and improves oxygenation by delivering high oxygen content gas into the endotracheal region of a patient. The apparatus is generally termed a "breathing tube" or "endotracheal tube" ("ETT"). Respiratory failure is the primary indication for use of such respiratory support systems, including hypoxemic respiratory failure, as may result from pulmonary conditions such as pneumonia, pulmonary edema, pulmonary hemorrhage and respiratory distress syndrome, and hypercarbic respiratory failure, such as may be associated with disease states causing inadequate alveolar ventilation to meet metabolic demands. Respiratory support systems are also necessary during many surgical procedures. Mechanical ventilation support systems are known in the art and described, for example, in U.S. Pat. No. 4,502,482 to DeLuccia, deceased et al., U.S. Pat. No. 5,005,573 to Buchanan, U.S. Pat. No. 5,452,715 to Boussignac et al., U.S. Pat. No. 5,606,968 to Mang, U.S. Pat. No. 5,765,559 to Kim, U.S. Pat. No. 5,791,338 to Merchant et al., U.S. Pat. No. 5,803,898 to Bashour, and U.S. Pat. No. 5,806,516 to Beattie.
For seriously ill patients who require mechanical ventilatory support and are thus already burdened with an endotracheal intubation system, introduction of an additional system for PCO.sub.2 measurement--such as described in U.S. Pat. No. 5,579,763, U.S. Ser. No. 09/160,224 and PCT Application No. PCT/US98/20118--would be logistically difficult and unnecessary if the measurement could be determined using the existing endotracheal system.
There is a accordingly a need in the art for a method to measure perfusion failure and to monitor the effectiveness of methods taken to increase perfusion, e.g., blood infusion or the like, that are well-suited for use in a mechanically ventilated patient.